#include<iostream>
#include<algorithm>
using namespace std;

int main(){
	int n;
	scanf("%d\n",&n);
	string ans;
	for(int i = 0; i < n; i++){
		string s;
		getline(cin, s);
		int lens = s.length();
		reverse(s.begin(), s.end());
		if(i == 0){
			ans = s;
			continue;
		}else{
			int lenans = ans.length();
			if(lens < lenans) swap(ans, s);
			int minlen = min(lens, lenans);
			for(int j = 0; j < minlen; j++){
				if(ans[j] != s[j]){
					ans = ans.substr(0,j);
					break;
				}
			}
		}
	}
	reverse(ans.begin(), ans.end());
	if(ans.length() == 0) ans = "nai";
	cout << ans;
	return 0;
} 
